The present invention relates to a method according to the preamble of claim 1 for producing thin films.
In the present method, a substrate located in a reaction space is subjected to alternately repeated surface reactions of at least two different reactants used for producing a thin film. The vapor-phase reactants are admitted repetitively and alternately each reactant from its own source into the reaction space where they are allowed to react with the substrate surface for the purpose of forming a solid-state thin film product on the substrate. Reaction products which have not adhered onto the substrate and any possible excess reactant are removed from the reaction space in vapor phase.
The invention also concerns an apparatus according to the preamble of claim 8.
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other corresponding vacuum deposition methods, different variants of the Chemical Vapor Deposition (CVD) method (including low-pressure and organometallic CVD and plasma-enhanced CVD), or alternatively, the above-described deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy (ALE) method. In the MBE and CVD methods, besides other process variables, the thin-film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform thickness of the layers deposited by the first category of conventional methods, the concentrations and reactivities of the starting materials must be kept equal all over the substrate area. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, for instance, the risk of their mutual reaction arises. Then, the risk of microparticle formation already in the inflow channels for the gaseous reactants is imminent.
Such microparticles usually have a deteriorating effect on the quality of the thin film. Therefore, the possibility of premature reactions in MBE and CVD reactors is avoided by heating the starting materials no earlier than at the substrate surfaces. In addition to heating, the desired reaction can be initiated using, e.g., a plasma or some other similar activator.
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the ALE process is based on allowing the substrate surface qualities rather than the starting material concentrations or flow properties to control the deposition rate. The only prerequisite in the ALE process is that the starting material is available in sufficient concentration for thin-film formation all over the substrate.
The ALE method is described in, e.g., FI Patent Specifications Nos. 52359, 97730 and 57975, in WO Publication No. 96/17107 and in U.S. Pat. Nos. 4,058,430 and 4,389,973, in which also some apparatus embodiments suited for implementing this method are disclosed. Apparatus constructions for growing thin films can also be found in the following publications: Material Science Report 4(7) (1989), p. 261, and Tyhjixc3x6tekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261.
In the ALE deposition method, atoms or molecules are arranged to sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon. According to the conventional techniques known from FI Patent Specification No. 57975, the saturation step is followed by an inert gas pulse forming a diffusion barrier which sweeps away the excess starting material and the gaseous reaction products from the substrate. The successive pulses of different starting materials and of diffusion barriers of carrier gas, the latter separating the former, accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. Such reactor is called the xe2x80x9ctraveling-wavexe2x80x9d reactor. To the function of the process it is irrelevant whether it is the gases or the substrates which are moved, but rather, it is imperative that the different starting materials of the successive reaction steps are separated from each other and arranged to impinge on the substrate alternately.
Most vacuum evaporators operate on the so-called xe2x80x9csingle-shotxe2x80x9d principle. Hereby, a vaporized atom or molecule can impinge on the substrate only once. If no reaction with the substrate surface occurs, it is rebound or re-vaporized so as to hit the apparatus walls or the vacuum pump undergoing condensation therein. In hot-walled reactors, an atom or molecule impinging on the reactor wall or the substrate may become re-vaporized and thus repeatedly impinge on the substrate. When applied to ALE reactors, this xe2x80x9cmulti-shotxe2x80x9d principle can provide, i.a., improved efficiency of material consumption.
If the starting materials in ALE deposition, due to flow dynamic or other reasons, are unevenly distributed over different parts of the substrates, it is necessary to pulse each starting material over the substrates in an amount which will guarantee that even at the thinnest flow, a sufficient amount flows at each pulse in order to ensure an even growth of the film. Knowing, on the other hand, that flow geometry can lead to concentration differences of even several decades, it may in the case of a disadvantageous flow geometry be necessary to pulse greater amounts of starting materials than the growth of the film in its entirety would presuppose. This is termed overdosage and other reasons may also exert an influence here, such as the chemistry of the starting materials.
In order for a sufficient amount of starting material to be provided over different parts of the substrate without any significant overdosage, two solutions are applied to achieve an even distribution of the gases:
1. The apparatus is constructed such that the pressure on the substrates is so low that the average mutual collision distance of the gas molecules is greater than the distances between the substrates. Hereby most of the collisions of the gas molecules will hit the substrates and few gases are evenly distributed with regard to the substrates. When the average collision distance is equal to the distance d between walls in a typical system or at least one hundredth part thereof, the gas is called transitional. At a pressure of one millibar and at room temperature the collision distance of one nitrogen molecule is 64 micrometers and at 0.01 mbar, 6.4 mm. In ALE reactors the distances between the substrates are typically in the range of a few millimeters. Thus, when the discussed solution is sought, the pressure must be about 1 mbar or preferably even lower.
2. At a greater pressure the average collision distance of the gas molecules is diminished and the gas is no longer transitional but is instead in a viscous state (collision interval less than d/100). In viscous state the flow of the gas is collective movement of its molecules, linked together via the collisions, towards the decreasing pressure. The intermixing of the molecules caused by the thermal movement is expressed as diffusion. In these solutions the aim is to distribute gas evenly over the substrates by means of different gas flow generators and nozzles because the diffusion velocities in the cross direction of the gas flow are small compared to the speed of propagation of the gas.
The problem with the first alternative is the low pressure required in the case of real dimensions. When the pressure decreases by a decade, the pump capacity must also be increased by a decade and the gas velocities are also increased by a decade if the gas flow is kept constant.
The gas velocities cannot surpass acoustic velocity and the pump prices are drastically increased. In addition, with the decreasing pressure the dimensions of the reactor must be enlarged in order to achieve improved transmittance such that pressure losses increasing the pressure are prevented. This would again presuppose that the pressure be reduced.
The pressure can also be reduced by reducing the sweep flows and the transmission flows of the starting materials. As there is still the same amount of starting material to be transmitted and sufficient sweeping is required, this will result in prolonged process times. This does not necessarily pose a problem is research reactors but the problem will be accentuated in production apparatuses and in the case of large substrate surface areas.
In the solution according to the second alternative the pressure is typically 2 to 10 mbar whereby the pumps needed are of moderate size, the dimensions of the piping and the substrate holder easy to implement and the flow times and rates of the gases reasonable.
When gases are distributed evenly over the entire breadth of the substrates, different kinds of collision causers and pressure reducing devices or throttles are used. In collision causers the gas jet is made to impinge on a surface which causes diffusion and mixing of the jet. It is also possible to arrange several successive collisions like this. The function of the gas throttles, then, is based on the fact that on the entering side of parallel throttles there is great conductance compared to the conductance through the throttles. Thus, for the gas all throttles are equivalent and the flow is evenly distributed into linear or planar form. As throttles, e.g. a narrow slit, adjacent holes (pipes), sinter, or the like, may be used.
Even though an even flow has been achieved, in practice there have been problems with the evenness of the film because viscous gases are directed towards the lower pressure, i.e. towards the gas exhaust port.
A solution to this problem has been described in FI Patent Specification No. 97731. In the solution according to the patent the evacuation of gas is regulated by controlling the throttle at the exhaust end. The method is functional when the channels at the feed end are large and dynamic factors caused by intense xe2x80x9cgas jetsxe2x80x9d cannot affect the distribution. Therefore, in spite of an even suction at the exhaust end even the infeed flows have been flattened, thereby seeking to assist the distribution of the gas mixture.
The evenness of the exhaust suction has been achieved such that the conductance of the exhaust duct is, for example, 10 to 100 times the conductance of the exhaust slits. In some known solutions the slit between the substrates functions as a throttle dividing the flows. Thus, the gas flow distribution can be influenced by the relation between the exhaust duct conductance and the exhaust slit conductance. Once the gases have passed the exhaust slit their possible intermixing is no longer of any great significance, and thus, the conductance of the exhaust duct can be increased by increasing the dimensions of the exhaust duct. Another alternative for achieving a sufficiently good conductance ratio would be to reduce the conductance of the exhaust slits. The problem would be posed, however, of increased pressure in the reactor and also in the delivery piping.
As regards the input side, then, the gas flow rate, the sweepability of the channels and pressure losses play a major role. If the conductance ratio is controlled by choking the feed slits, the pressure in the feed channel will rise, causing the lowest possible pressure at a solid (or liquid, gas) source to rise. This, then, presupposes that the source temperature be raised in order to raise the vapor pressure (if possible), which in turn may have a negative effect on the starting material. (E.g. the efficiency of the source (magnitude of supplied starting material pulse as compared to the amount of carried gas supplied) is dependent on the ratio between the pressure in the starting material vessel and the vapor pressure of the source material at the temperature in question. With an increasing pressure the collisions between the molecules are increased which for some starting materials will lead to decomposition). With an increased pressure a denser gas results, and thus the flow rates in the piping may be reduced to a harmfully low level. The conductance ratio may also be affected by increasing the size of the feed channels. This will reduce the flow rates and a slow reactor results.
In the construction according to the invention the delivery piping is constructed of pipings branched in a tree-like manner. Modeling software has been developed for calculating the piping and can be used to dimension the piping such that gases are evenly distributed even at a small conductance ratio, whereby the pack can be designed rapid, causing only small pressure loss such that there is little variation of the gas velocity in the feed channels (i.e. the different parts are well swept, the starting material is not unnecessarily hammered by accelerations and decelerations).
In previous constructions (where the choking is taken care of by other means than the slit between the glasses) the substrate was placed in a recess provided in a plate parallel with the substrate. Hereby part of the delivery and exhaust piping is provided in the plate per each substrate or pair of substrates. With an increased number of substrates also the number of these plates which are larger than the substrate is correspondingly increased. The problem here is that the price and weight of the pack are increased in direct proportion when the number of substrates in increased. (The plate is about 10 to 15 cm longer and about 5 to 10 cm broader than the substrate and it must be machined with very strict tolerances. As the material, for instance titanium has been used, whereby the mass of this plate supporting the substrate is great in relation to e.g. the weight of glass or Si wafer substrates). The great weight also increases masses which are slowly heated in vacuum. The known structure is compact and in the construction the pack can be constructed with few parts but with increasing numbers of substrates the afore-cited problems become considerable.
In the construction according to the invention a separate gas supply unit is constructed for feeding the gases. It could be assembled of pipes but is most advantageously formed of sheets into which gas slits are machined. The sheets are placed transversely with regard to the substrates. Hereby a system of gas supply channels is formed in the sheets whose surface area is slightly greater than the cross-section of the pack.
It is an object of the present invention to overcome the drawbacks of conventional technology and to provide an entirely novel method and apparatus for growing thin films of homogeneous quality.
The invention is based on the notion of constructing a tree-like inflow manifold and the plane of the feed slits is preferably essentially perpendicular to the plane of the substrates.
The method of the invention can be advantageously implemented in an apparatus having a pyramid-like cassette structure which can be pulled out of the reaction space in its entirety or, alternatively, one pair of substrates, one pyramid or V structure at a time.
More specifically, the method according to the invention is principally characterized by what is stated in the characterizing part of claim 1.
Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 8.
In the context of the present invention, the term xe2x80x9creactantxe2x80x9d refers to a vaporizable material capable of reacting with the substrate surface. In the ALE method, reactants belonging to two different groups are conventionally employed. The reactants may be solids, liquids or gases. The term xe2x80x9cmetallic reactantsxe2x80x9d is used of metallic compounds or even elemental metals. Suitable metallic reactants are the halogenides of metals including chlorides and bromides, for instance, and organo-metallic compounds such as the thd complex compounds. As examples of metallic reactants Zn, ZnCl2, Ca(thd)2, (CH3)3Al and Cp2Mg may be mentioned. The term xe2x80x9cnonmetallic reactantsxe2x80x9d is used for compounds and elements capable of reacting with metallic compounds. The latter group is typically represented by water, sulphur, hydrogen sulphide and ammonia.
In the present context, the term xe2x80x9ccarrierxe2x80x9d gas is used to refer to a gas which is admitted into the reaction space and is capable of preventing undesired reactions related to the reactants and, correspondingly, the substrate. Such reactions include e.g. the reactions of the reactants and the substrate with possible impurities. The carrier gas also serves to prevent reactions between substances of different reactant groups in, e.g., the inflow piping. In the method according to the invention, the carrier gas is also advantageously used as the carrier gas of the vapor-phase pulses of the reactants. According to a preferred embodiment, in which reactants of different reactant groups are admitted via separate inlet manifolds into the reaction space, the vapor-phase reactant pulse is admitted from one inflow channel while the carrier gas is admitted from another inflow channel thus preventing admitted reactant from entering the inflow channel of another reactant. Of carrier gases suited for use in the method, reference can be made to carrier gases such as nitrogen gas and noble gases, e.g., argon. The carrier gas may also be an inherently reactive gas such as hydrogen gas serving to prevent undesirable reactions (e.g., oxidization reactions) from occuring on the substrate surface.
According to the invention, the term xe2x80x9creaction spacexe2x80x9d includes both the space in which the substrate is located and in which the vapor-phase reactants are allowed to react with the substrate in order to grow thin films (namely, the reaction chamber) and the gas inflow/outflow channels communicating immediately with the reaction chamber, said channels serving for admitting the reactants into the reaction chamber (inflow channels) or removing the gaseous reaction products of the thin-film growth process and excess reactants from the reaction chamber (outflow channels). According to the construction of the embodiment, the number of the inflow and outflow channels, respectively, can be varied from one upward. They may also be located at opposite ends of the substrates whereby the outflow orifice corresponding to each reactant group is located at the end of the inflow of the other group, advantageously separated therefrom by means of a tongue (cf. the embodiment of FIG. 3). The gases can be fed onto the substrate alternately from opposite directions. In this manner it is possible to compensate any observed stronger film growth at the inflow end of the substrate. Also the exhaust suction from the outflow channel is arranged in an alternated manner.
Herein, the term xe2x80x9csubstrate surfacexe2x80x9d is used to denote that top surface of the substrate onto which the vapor-phase reactant flowing into the reaction chamber first impinges. In practice, said surface, during the first cycle of the thin-film growing process is constituted by the surface of the substrate such as glass, for instance; during the second cycle the surface is constituted by the layer formed during the first cycle and comprising the solid-state reaction product which is deposited by the reaction between the reactants and is adhered to the substrate, etc.
According to the invention the flow over the individual substrates is determined by the flow at the inflow end because a flow of as even a velocity as possible is advantageous particularly due to the dynamic properties of the flowing material.
In order to grow an even film it has been found essential in the solution of the invention that
Each reactant group is fed directly into the reaction chamber via a separate inflow channel. Preferably, the reactant is allowed to become mixed and homogenized with a carrier gas flow entering from the inflow channel of another reactant group prior to contacting the reactant with the substrate. The exit ends of the inflow channels of the different reactant groups, later in the text called the reactant xe2x80x9cfeed orificesxe2x80x9d, are adapted to exit into the reaction chamber, close to the substrates of the thin film structures. To eliminate the risk of reactant contamination, a carrier gas flow is hereby particularly advantageously driven through that inflow channel or channels which is/are currently not used for feeding a reactant. The reactant feed orifices are arranged on opposite sides with regard to the feed orifices for carrier gas.
The homogenization zone may also be arranged prior to the reaction chamber.
In the reaction zone, the aim is to maintain the homogeneity of the gas flowing through. By constructing a narrow reaction chamber (i.e. such that at the substrates its height is small in relation to its width) no concentration profile is created in the gas flow in the reaction chamber. The reaction chamber enclosing the substrate is particularly advantageously designed to have the chamber walls disposed close to the substrate being processed.
As an alternative to the present invention, the choking can be implemented as a narrow suction slit between the exhaust channel communicating with a negative pressure source (e.g. a vacuum pump) and the reaction chamber. This may be constituted by one continuous slit but also of many small parallel slits preceded by a reaction chamber having a great conductance with regard to the slits.
An apparatus having a cassette-type structure is provided with carrier gas sealing which is implemented such that a suction groove is formed in the surface of the planar elements following their perimeter close to the edges of the plates to collect any leaks. The suction groove is made to communicate with the negative-pressure outflow channel. The purpose of the suction groove is to avoid the access of external contamination into the reaction space and to prevent reactants from leaking outside the reaction space. The isolating seal flow in the suction groove functions optimally if the strongest constriction of the reaction gases takes place at the end of the substrate, close to the outflow channel. It is essential for the operation of the suction groove that the pressure in the suction groove is lower than in the reaction space or outside it.
The parts of the cassette structure are made of a material whose surfaces are inert with regard to the reactants used in ALE deposition. Advantageous materials include glass and similar silicate-based materials as well as different ceramic materials.
The solution according to the invention provides considerable benefits over the previously employed ALE solutions. Thus, because the distribution of the gases onto the substrate surfaces becomes more even, a smaller overdosage becomes possible, resulting in savings in the amounts of starting materials and a shorter process time. The quality of the film is also improved. As a correctly dimensioned tree-like structure can be used to reduce pressure losses, the system pressure at the sources is reduced, which increases the efficiency of the apparatus. An inflow manifold dimensioned in accordance with the invention will also provide a swifter apparatus which in turn results in shorter process times.
The above-described apparatus construction details make it possible to reduce the weight of the reaction space and to minimize the number of components in the system.